Mountain sandwort in decline on Ben Lawers NNR

Mountain sandwort (Sabulina rubella) is among the rarest of our arctic-alpine plants. It’s scattered across a handful of Scottish mountains, from Ben Lawers in the south to Ben Hope in the north, with populations also found on Ben Alder and the Cairnwell. Ben Lawers holds by far the most plants, from Meall Corranaich in the west of the range, to the jumble of crags and boulders which lie close to the summit of Ben Lawers itself. All of the sites are over 900m, whereas further north it occurs at lower altitudes, being found at around 450m on Ben Hope and previously down to 120m on the serpentine rocks of Unst, Shetland, where it was last seen in the 1950s.

It’s a small cushion-forming plant, with narrow leaves overtopped by delicate white flowers on slender stems. Typically it grows on base-rich crags, slabby boulders and occasionally in exposed patches of gravelly soil within grassland.

The first census on Ben Lawers took place in 1981, and monitoring has been repeated every few years since. For many years mountain sandwort gave no concern, and the number of known plants increased, partly as new populations were discovered, reaching a peak of 2,383 in 2006. Use of GPS and digital photography has led to greater confidence in the accuracy of repeat monitoring, as the same locations are easily found again.

When monitoring took place in 2012, the count had declined to 1,799. This gave no immediate cause for concern, as the places where mountain sandwort grows are often dynamic – vegetation can increase, leading to plants intolerant of competition declining, while new ground for colonisation can be opened up by landslips and rockfalls. However, the monitoring in 2020 showed a further decline, with a total of 998 plants counted. Numbers have declined at almost every site, with no plants at all found on the Meall nan Tarmachan range, to the west of Ben Lawers, where it had only ever been known in low numbers. The overall decline from 2006 to 2020 is consistent, averaging at roughly 100 plants per year.

What about the other mountains where it is found in Scotland? All of them are Sites of Special Scientific Interest (SSSI), and NatureScot (previously Scottish Natural Heritage) is responsible for ensuring that monitoring of important features takes place. As a result, there have also been regular counts of mountain sandwort on Ben Alder, the Cairnwell and Ben Hope. The results at these sites mirror what has been found on Ben Lawers, with numbers declining from 69 in 2003 to 25 in 2013 on Ben Alder; 103 in 2011 to 18 in 2017 on the Cairnwell; and over 50 in 1995 to an alarmingly low 7 in 2012 on Ben Hope. As mentioned earlier, it disappeared from Unst in the 1950s, with the finger of blame being pointed at collectors.

So what’s going wrong for this plant? Collecting is very largely a thing of the past, but the effects of climate change are increasing. Comparing the site photographs from different years indicates that, at some sites, other vegetation is pushing out the mountain sandwort. A common and often-vigorous moss, particularly on boulders, is Racomitrium lanuginosum, also known as woolly fringe moss. Mountain sandwort appears to be able to tolerate some competition from this species, but too much and it disappears. The healthiest populations of mountain sandwort were found on sloping slabs growing from a mat of small liverworts and mosses; on the single best slab it was accompanied by a range of rarities including snow gentian (Gentiana nivalis), rock speedwell (Veronica fruticans), alpine pearlwort (Sagina saginoides) and alpine saxifrage (Micranthes nivalis). The largest plants with the most seed capsules were always found where competition was low.

Some sites had suffered from recent landslips that had removed plants. In time, this bare ground may be recolonised, but no evidence of this was found during the monitoring. However, in many places there was no obvious reason for a decline, with many of the plants present bearing seed capsules and growing in suitable habitat.

So what can we do when our monitoring shows that a rare species is in decline? When highland saxifrage (Saxifraga rivularis) was on the verge of disappearing from Ben Lawers, we had good evidence that it was as a result of historic collecting. With this threat having disappeared, seeds were collected from plants at Glencoe, grown on and planted out around the last surviving plant, which died not long afterwards. The highland saxifrage plants are now reproducing by seed and doing well. We know montane willows, such as woolly willow (Salix lanata), cannot tolerate browsing, so we grew plants on in our nursery and planted them out in exclosures. However, when the cause of the decline is not obvious, particularly when it may be linked to climate change, it’s more difficult to meaningfully intervene.

In the first instance, we’ll be working with the Royal Botanic Garden Edinburgh to gather seeds for an ex-situ collection. In a way, this is an insurance policy against the worst-case scenario where the plant disappears entirely, in the hope that at some point in the future we may understand enough to be able to reintroduce it. We’ll also be working with NatureScot to encourage more research, in an attempt to find out what is going on. Part of this research will include monitoring – when a species is declining as rapidly as the mountain sandwort, the frequency of monitoring needs to be increased, so instead of an eight-year interval we will monitor again in 2023. Hopefully, the trend will be bucked and we see a recovery!